textview: make magnifier size dependent on text size

The magnifier is now set enough height to show the line being currently
manipulated, and the shown coordinates are made relative to the iter
position on the Y axis so it always guarantees to show the full line.

The popover positioning (and the X axis of the shown content) are made
relative to the touch position, with an extra margin accounting for the
finger, similarly to how it was done previously.
textview: set magnifier magnification to 1.5x